1. Field of the Invention
The present invention relates to a development technique by which a developing roller is used to supply a developer to a photosensitive surface of a photosensitive material, and particularly it relates to a technique of suppressing toner adhering to an out-of-image portion in dust, i.e. the occurrence of a so-called image fouling.
2. Description of the Related Art
As for a development technique of using a developing roller to supply a developer to a photosensitive surface of a photosensitive material thereby to make an electrostatic latent image formed on the photosensitive surface visible, there has been known a technique of supplying a developing bias voltage having DC and AC components to the developing roller conventionally. The DC component of a developing bias voltage herein serves to make the developer adhere to the photosensitive surface. Also, the AC component of the developing bias voltage herein serves to force the developer adhering to the photosensitive surface to vibrate thereby to make clear the boundary between a region in the photosensitive surface where an electrostatic latent image is formed and a region with no latent image formed therein.
Conventionally, there has been known a technique such that a frequency range of AC component is set to the peak voltage value width of AC component or larger, for example, 1500 Hz or higher when the peak voltage Vp-p is set to 1500V, thereby to suppress the image fouling and improve the quality of an image (JP-A-2002-351198).
Further, when e.g. a circumferential speed N of a developing roller (magroller) is 300 mm/sec in the case where 10000 Hz is used at the time of a normal speed, the AC component reciprocates 100/3 times per millimeter. Therefore, there has been known a technique that in reducing the speed to 150 mm/sec while maintaining the relation, 100/3×150=5000 Hz and as such, the frequency is used for slowdown.
However, as in JP-A-2002-351198, in the case where the frequency f is set to 1500 Hz or higher when the peak voltage Vp-p is set to 1500V, the upper limit in setting the frequency is lost. In the case where there is no upper limit for the frequency, a phenomenon that an image and a character are dissipated on a photosensitive material when the frequency is excessively high will be caused. Further, in the case where the frequency is set to 1200 Hz when the peak voltage Vp-p is 1200V, “fouling characteristic” is not made satisfying. Satisfying “fouling characteristic” can not be achieved until the frequency is set to be a high frequency of 10000 Hz or higher. In other words, fouling characteristic can not be made satisfying until the frequency is made higher than the frequency lower limit condition defined in the above-described conventional technique (JP-A-2002-351198).
Further, in the case where the frequency of AC voltage is set to 10000 Hz when the circumferential speed N of the developing roller is 300 mm/sec and the fouling characteristic is satisfying with no dust, using the frequency, 100/3*150=5000 Hz, in reducing the circumferential speed N to 150 mm/s while maintaining a relation such that the AC component reciprocates 100/3 times per millimeter of the developing roller worsens the fouling characteristic. Moreover, in such case, a satisfying fouling characteristic can not be achieved until the frequency is set to a higher one. Still further, increasing the frequency of the developing bias voltage excessively increases the reciprocating motion (vibration) of the developer on the photosensitive surface and causes an image formed on the photosensitive surface by the developer to be dissipated, which can lead to degradation of image quality.